Colicin Ib

Colicin Ia is a type of Colicin, a bacteriocin made by E. coli which acts against other nearby E. coli to kill them by forming a pore in the membrane, leading to depolarisation of the membrane which kills the cell.

Synthesis and release
The operon for colicin Ib is encoded on a plasmid in the cytoplasm of the E. coli. This operon also encodes its Colicin Immunity Protein, Imm, to protect the membrane of the cell from the pore forming activities of the colicin, alongside a protein to aid the release of the colicin outside the cell.

Once produced, the immunity protein inserts into the membrane to prevent pore formation.

Mechanism of uptake
The receptor binding domain of the colicin binds to the outer membrane receptor Cir on the target cell; it is a constitutively expressed protein parasitised by the colicin. The translocation domain then recruits proteins from the Ton system to translocate the protein across the membrane and into the cytoplasm.

Killing Activities
Colicin Ib has Pore Formation activities, whereby it inserts into the membrane of the targeted cell from the cytoplasm, and forms a pore. This causes depolarisation of the membrane, which is needed for the cell to synthesise ATP. Once this process is halted, the cell cannot survive and so dies.